1. Field of the Invention
This invention relates to a CaCO.sub.3 -talc coating pigment slurry, to a process of preparing the same, to its preferred uses, and to a novel adjuvant combination for use in grinding the pigments.
2. Description of the Prior Art
Coated papers are coated with a composition which essentially comprises a binder and a pigment, which consists of a mineral filler. A description of the components of compositions for coating paper and of their use can be found, e.g., in the book by James P. Casey "Pulp and Paper Chemistry and Technology", Chapter XIX, Volume III (1961).
Three different processes are known in the printing art, namely, letterpress printing, offset printing, and gravure printing.
Letterpress printing is a relief printing process. As in a stamp, the printing surface is a raised surface portion of the printing plate. The raised image surface is inked by means of a roller. Coated paper, which contains kaolin or calcium carbonate, may be used in that printing process.
Offset printing is a planographic printing process, in which the printing and non-printing surfaces lie in a plane and are distinguished from each other in that the printing surface portions are adapted to receive and release grease (i.e. printing ink), but repell water, whereas the non-printing surface portions retain water but repell grease (ink). The rotating printing cylinder first contacts the water-carrying damping roller and subsequently the ink-carrying inking roller. But the printing cylinder does not print directly on the paper but on a rubber blanket, which is trained around a cylinder, from which the printed image is transferred to the paper, which is urged by a cylinder against the blanket cylinder. Coating pigments which contain kaolin and calcium carbonate are used in that process.
In gravure printing, the printing portions of the cylinder are disposed below the polished surface. The cylinder is first moved through liquid ink in a trough and subsequently contacts a doctor blade, which is a steel blade, by which surplus ink is scraped off whereas the ink for printing is retained in small cups until the paper is urged by an impression cylinder against the printing cylinder and sucks the ink out of the cups.
Kaolin is predominantly used as a coating pigment in the gravure printing practice. But the use of kaolin as a coating pigment in papers for gravure printing involves a number of disadvantages and particularly has an unsatisfactory rheological behavior. For this reason it is not possible to use coating compositions having a solids content so that the drying of the coating involves high energy costs. Moreover, kaolin-based coating pigments in papers for gravure printing have only very low whiteness values.
It is known in the art to use a so-called "Superclay" as a coating pigment in papers for gravure printing.
The use of talc as a coating pigment has already been investigated. For instance, talc has been used as a filler in papers in Finland since the 1960's. The chemical structure of talc is similar to that of kaolin. Kaolin consists of aluminosilicate platelets. Talc is also a pigment consisting of platelets, but consists of magnesium silicate. Besides, the most significant physical properties of talc do not substantially differ from those of kaolin.
But the use of talc in compositions for coating paper also involves various disadvantages. Owing to its chemical structure, talc is hydrophobic and for this reason the pigment coating can be wetted by dispersions only with difficulty and an effective venting will be required during the application of dispersions. Moreover, the use of talc rather than kaolin in the coating machine will necessitate the use of a novel type of binders and a different mode of operation of the coating machine in order to ensure that the retention of water, the air content and the rheology will not give rise to major problems.
Owing to said disadvantages it has already been attempted to use mixed pigments consisting of kaolin and talc in coating compositions for papers for gravure printing and for web-fed papers for offset printing. (See Tappi Journal, Volume 68, No. 11, pages 92 and 93, November 1985).
From "Wochenblatt fur Papierfabrikation", 107, pages 909 to 914 (1979), No. 23/24, it is already known to add 5 to 10% talc to coating compositions containing calcium carbonate for use on papers for offset printing.
From Tappi Journal, Volume 68, No. 11, page 93, November 1985, it is also known to use mixed pigments consisting of 85% talc and 15% calcium carbonate. It is very difficult to grind and process talc alone in the preparation of slurries because the product is highly hydrophobic. For this reason 15% CaCO.sub.3 have been added to the talc in the preparation of a slurry having a higher solids content merely in order to improve the processing but without an intention to make improved coating pigment compositions. As regards the properties of papers used for gravure printing such a mixture of 85% talc and 15% CaCO.sub.3 will have no advantages over a mixture of talc and kaolin and there is even a risk that the still low solids content of the coating composition (CaCO.sub.3 will not have a favorable rheological influence unless it is added in an amount of or above 50%) may decrease the gloss of paper. Severe problems will arise in the processing of such papers owing to the slipperiness of the surfaces coated with such compositions and the sliding properties of said surface.
EP 0 332 102 A2 discloses mixed pigments consisting of 40 to 80% by weight calcium carbonate and/or dolomite and 20 to 60% by weight talc or 20 to 60% by weight of a talc-kaolin mixture or 20 to 60% by weight of a talc-mica mixture. That mixture is prepared in that the several components are mixed. It may be used in the papermaking industry, particularly as a coating composition for papers for gravure printing. That known pigment mixture has particularly the following disadvantages:
Poor dispersibility owing to the hydrophobic nature of talc; PA1 inhomogeneities due to talc agglomerates in the mixed slurry so that irregularities are found in the coating; PA1 from the examples it is apparent that only low solids contents are achieved. PA1 a. 24 to 64% by weight CaCO.sub.3, PA1 b. 5 to 48% by weight talc, PA1 c. 20 to 40% by weight H.sub.2 O, and PA1 d. an adjuvant combination consisting of PA1 a. 95 to 100% particles &lt;10 .mu.m, PA1 b. 50 to 95% particles &lt;2 .mu.m, PA1 c. 27 to 75% particles &lt;1 .mu.m, PA1 d. 0.1 to 35% particles &lt;0.2 .mu.m, PA1 a. 28 to 61% by weight CaCO.sub.3, PA1 b. 10 to 44% by weight talc, PA1 c. 22 to 38% by weight H.sub.2 O, and PA1 d. an adjuvant combination consisting of PA1 a. 33 to 56% by weight CaCO.sub.3, PA1 b. 14 to 37% by weight talc, PA1 c. 25 to 36% by weight H.sub.2 O, and PA1 d. an adjuvant combination consisting of PA1 a. 39 to 51% by weight CaCO.sub.3, PA1 b. 18 to 31% by weight talc, PA1 c. 27 to 33% by weight H.sub.2 O, and PA1 d. an adjuvant combination consisting of PA1 a. 44 to 48% by weight CaCO.sub.3, PA1 b. 21 to 25% by weight talc, PA1 c. 28.2 to 32.2% by weight water, and PA1 d. an adjuvant combination consisting of PA1 a. 96 to 100% particles &lt;10 .mu.m, PA1 b. 60 to 92% particles &lt;2 .mu.m, PA1 c. 35 to 70% particles &lt;1 .mu.m, PA1 d. 3 to 28% particles &lt;0.2 .mu.m, based on the equivalent diameter of each particle. PA1 a. 98 to 100% particles &lt;10 .mu.m, PA1 b. 68 to 85% particles &lt;2 .mu.m, PA1 c. 42 to 62% particles &lt;1 .mu.m, PA1 d. 9 to 22% particles &lt;0.2 .mu.m, PA1 a. 99% particles &lt;10 .mu.m, PA1 b. 78% particles &lt;2 .mu.m, PA1 c. 53% particles &lt;1 .mu.m, PA1 d. 28% particles &lt;0.2 .mu.m,
For this reason it has already been proposed to jointly grind calcite and talc. DE 33 12 778 C2 discloses such a process, in which an aqueous suspension containing talc powder and calcite powder is microground by attrition grinding. In that case it is preferred to use a mixture of talc, calcite, and water with a solids content of 50 to 80%. Talc and calcite are preferably used in a mixing ratio from 2:1 to 1:3.
The assignee of the present invention has attempted to use that process although DE 33 12 778 C2 fails to disclose even a single example. It has been found that it is not possible to achieve the required solids content.
For this reason the assignee of the present invention has attempted to use the conventional grinding aids in order to achieve improved results. Such grinding aids are known, e.g., from DE 27 48 243 A1, which relates also to a talc pigment. In accordance with DE 27 48 243 A1 talc is contacted with a surfactant, a polyelectrolyte and a defoamer or with carboxycellulose and a polyelectrolyte and optionally with other additives during or after the granulation of the talc and is slurried in water or in solution of one or more of said substances.
But it has been found that in spite of the use of known grinding aids the process disclosed in DE 33 12 778 C2 had resulted in an end product having a high grey level and very high viscosities were measured immediately after the grinding and it was not possible to ensure an adequate shelf life.
It is finally known from "Wochenblatt fur Papierfabrikation" 1991, No. 17, pages 662 to 665, that mixtures of talc and carbonate may be used to make effectively printable papers for gravure printing if the coating composition is applied by means which permit the coating machines to be operated at a higher velocity and with a low frequency of web breaks.